1. Field of the Invention
The present invention relates to a method for measuring distortion.
2. Description of the Related Art
A method for simultaneously minimizing high-frequency and low-frequency components of a residual signal with respect to a general image is used in a motion estimation method according to related arts. A method for obtaining residual signals between a current block and a reference block obtained by the motion estimation method is used in a motion compensation method. In the motion compensation method, a method for simultaneously minimizing the distortions of the high-frequency and low frequency components of the current block is generally used as a method for measuring distortion. The method may be implemented using Equation 1.
                    SAD        =                              Dist            Total                    =                                                    Dist                AC                            +                              Dist                DC                                      =                                          ∑                                  i                  ,                  j                                N                            ⁢                                                                                    C                    ⁡                                          (                                              i                        ,                        j                                            )                                                        -                                      R                    ⁡                                          (                                              i                        ,                        j                                            )                                                                                                                                              (        1        )            
Here, C(i, j) denotes a pixel value of the current block, and R(i, j) denotes a pixel value of the reference block. Also, i and j denote lateral and longitudinal positions in each of the blocks, respectively, and N denotes a size of each of the blocks. DistAC and DistDC denote a sum of distortion values of the high-frequency component and a sum of distortion values of the low-frequency component, respectively. SAD (Sum of Absolute Differences) refers to a method for measuring distortion widely used in existing MPEG-x, H.26x and the like. The residual signals between the blocks are obtained through the SAD to perform video coding.
Next, a method for minimizing the distortion value of the high-frequency component of the current block and the reference block is used as the method for measuring distortion. The method may be implemented using Equation 2. (For reference, see D. S. Turaga and T. Chen, “Estimation and mode decision for spatially correlated motion sequences,” IEEE Transactions on Circuits and Systems for Video Technology, Vol. 11, No. 10, pp. 1098-1107, October 2001).
                    MRSAD        =                              Dist                          A              ⁢                                                          ⁢              C                                =                                    ∑                              i                ,                j                                      ⁢                                                                                                (                                                                  C                        ⁡                                                  (                                                      i                            ,                            j                                                    )                                                                    -                                              m                        C                                                              )                                    -                                      (                                                                  R                        ⁡                                                  (                                                      i                            ,                            j                                                    )                                                                    -                                              m                        R                                                              )                                                                              .                                                          (        2        )            
Here, mC and mR denote a mean pixel value of the current block and a mean pixel value of the reference block, respectively. MRSAD (Mean-Removed SAD) is used together with a compensation method of DC-components so as to compress images with a large illumination change and multiview images. (For reference, see J.-H. Hur, S. Cho, Y.-L. Lee, “Adaptive Local Illumination Change Compensation Method for H.264/AVC-Based Multiview Video Coding,” IEEE Transaction on Circuits and Systems for Video Technology, Vol. 17, No. 11, November 2007).
Next, in the motion estimation method of images with a large illumination change, there has been proposed a technique for selectively applying a method based on the characteristics of images. In the method, in order to prevent an increase of the calculation amount generated in estimating only the high-frequency component of residual signals, an inaccurate high-frequency component of the current block is estimated using the mean value of non-neighboring pixels of the current block and the reference block, and an inaccurate low-frequency component of the current block is estimated using the non-neighboring pixels of the current block and the reference block in the motion compensation method. Equations 3 and 4 used in the motion compensation method are as follows.
b=mC−mR, assume that mC≈mCn and mR≈mRn then
                                          b            ~                    =                                    m                              C                n                                      -                          m                              R                n                                                    ,                                  ⁢                              b            ~                    ≈          b                                    (        3        )                                mMRSAD        =                              ∑                          i              =              1                        N                    ⁢                                    ∑                              j                =                1                            N                        ⁢                                                                          C                  ⁡                                      (                                          i                      ,                      j                                        )                                                  -                                  (                                                            R                      ⁡                                              (                                                  i                          ,                          j                                                )                                                              +                                          b                      ~                                                        )                                                                                                      (        4        )            
Here, mC and mR denote a mean value of the current block and a mean value of the reference block, respectively. Also, mCn and mRn denote a mean value of neighboring pixels of the current block and a mean value of neighboring pixels of the reference block, respectively. The approximate value of the mean value of each of the blocks, obtained as described above, is subtracted from pixel values of each of the blocks, so that a block having a high-frequency component most similar to that of the current block is detected from reference candidate blocks.
The combination of the neighboring pixels of the reference block and neighboring pixels of the current block for the purpose of the implementation, i.e. the combination of pixels used to evaluate mCn and mRn, has been designed. (For reference, see P. Yin, A. M. Tourapis, and J. Boyce, “Localized weighted prediction for video coding,” IEEE International Symposium on Circuits and Systems 2004, Vol. 5, pp. 4365-4368, May 2005).
However, in the related art method, characteristics of neighboring coded blocks are not used to measure the distortions of the high-frequency and low-frequency components, and hence, it is inefficient to enhance compression efficiency. Further, only the distortion of the high-frequency component is considered in the motion estimation, hence, there is a problem in enhancing only compression efficiency of specific images. (For reference, see D. S. Turaga and T. Chen, “Estimation and mode decision for spatially correlated motion sequences,” IEEE Transactions on Circuits and Systems for Video Technology, Vol. 11, No. 10, pp. 1098-1107, October 2001; J.-H. Hur, S. Cho, Y.-L. Lee, “Adaptive Local Illumination Change Compensation Method for H.264/AVC-Based Multiview Video Coding,” IEEE Transaction on Circuits and Systems for Video Technology, Vol. 17, No. 11, November 2007).
Furthermore, when the distortion estimation of the high-frequency component is performed in the motion estimation and a low-frequency estimation is performed after the motion estimation is finished, the distortion values of the low-frequency component caused in the low-frequency estimation are not considered. Hence, the compression efficiency is not significantly increased with respect to images with a small illumination change.